After an import of a DXF-file from Autocad(?) I tried to draw a line
between two points. The snapin worked good, but the line was
drawed 10-20 times to long, so it seems that is a problem with
scaling. It is possible to give the drawing the right scale in some way?

How did you conclude that the line is 10-20 times to long?
- Does it look too long on the computer screen?
- Is it printed too long on paper?
- Did you use a measuring tool in QCAD to measure the line length and the measurement was too long?

Note that CAD drawings are by definition unitless and scaled 1:1 (real size). Scaling and units only play a role for printing.